Synaptic transmission at the giant synapse in the CNS of the hatchetfish is under investigation. We are studying recovery of mPSP amplitude following a depressing tetanus which gives an estimate of time to fill synaptic vesicles. We are attempting to correlate the number of quanta releasable at the synapse with the number of vesicles estimated to have been present prior to release. Analysis of membrane recycling may allow direct estimates of actual number of vesicles released at a particular synapse, which then can be correlated with postsynaptic activity recorded. We hope to obtain more accurate measures of the number of vesicles responsible for a single postsynaptic quantal event. Iontophoretic application of transmitters and presynaptic intracellular injection of divalent ions are being carried out.